Rechargeable battery packs often include a protection circuit for controlling charging and discharging of a battery. The protection circuit monitors various electrical conditions of the battery such as charge level and direction of current flow. In response to the monitored electrical conditions, the protection circuit controls various switches for connecting and disconnecting the battery cells from the battery pack terminals. For example, the protection circuit monitors to detect overcharge and overdischarge conditions and, upon detection of such conditions, the protection circuit controls a switch which disconnects the battery from an external charger or an external load.
Typically, the circuitry of the battery pack, including the protection circuit, is powered by the battery. Unfortunately, a condition may arise in which the battery becomes discharged to such a level that the voltage is insufficient to power the protection circuit. In this condition, the protection circuit can no longer operate the switches for connecting and disconnecting the battery from the pack. Conventional protection circuits generally have protective circuits inside the battery pack which utilize a power switch such as one or more high power field effect transistors (FETs) connected in series with the battery. If the battery voltage falls below the minimum voltage for sustaining circuit operations, the switches are permanently turned off so that the battery cells are unalterably disabled from connection to the battery pack terminals.
Previously, permanent disabling of the battery was considered a feature because the behavior of a deeply discharged battery was not known. The general perception of those in the industry was that the charging of a deeply discharged battery was likely to be dangerous. As the rechargeable battery industry has matured, it has become known that battery cells can be charged from a very low level, safely and successfully. Thus, otherwise perfect and operational rechargeable battery cells are now rendered useless due to permanent disablement by conventional protect circuits.
Thus, there is a need for a circuit which maintains a capability to charge a completely discharged battery cell.
However, one disadvantage of such a circuit is that a battery with a potential difference of zero volts across the terminals may not be completely discharged. Instead, the battery may be short-circuited. In such a case, the circuit for allowing charging of a discharged battery could potentially attempt to charge the battery indefinitely.